The present invention generally concerns ceramic powders. It specifically concerns precursor materials or aggregates used to prepare ceramic powders via carbothermal synthesis. It particularly concerns precursor materials containing binder materials that are products of a reaction between at least one reactive hydroxyl moiety-containing carbonaceous compound and at least one reactive nitrogen moiety-containing compound.
Carbothermal synthesis or carbothermal reduction reactions commonly produce a variety of ceramic powders. The powders include nitrides such as aluminum nitride, silicon nitride and boron nitride, carbides such as silicon carbide, titanium carbide and boron carbide, and borides such as titanium diboride. The powders result from high temperature reactions between carbon and a metal oxide such as alumina, silica or titania. If a carbide is desired, the reaction typically takes place in the presence of an inert gas such as argon. If a nitride is desired, the reaction typically takes place in the presence of a nitrogen source such as gaseous nitrogen. If a boride is desired, a solid or gaseous source of boron must also be present.
Carbothermal synthesis reactions typically occur in any one of several conventional reactors. The reactions may be batch, semicontinuous or continuous. Known reactors include fixed bed reactors, pusher furnaces (U.S. Pat. Nos. 5,112,579 and 4,702,900), moving fixed bed reactors (U.S. Pat. Nos. 4,292,276 and 3,032,398), fluidized bed reactors (U.S. Pat. No. 5,108,713), rotary reactors (U.S. Pat. No. 4,368,181 and 3,802,847), and transport flow reactors (U.S. Pat. Nos. 5,190,737; 5,126,121; and 5,110,565; and PCT WO 90/08105 and 90/00276).
As a first step in a carbothermal reduction reaction, the carbon, metal oxide and any other solid reactants, catalysts or additives that may be required are typically converted into an intimate blend by wet or dry milling or blending procedures. Depending upon the type of reactor, the intimate blend may be used either as a dried powder or in the form of an aggregate. The aggregate typically contains a material that functions as a temporary binder. The binder material provides the aggregate with sufficient strength to allow handling during initial stages of a carbothermal synthesis reaction. As the reaction progresses to increasingly higher temperatures that are needed to carbothermally reduce the metal oxide, the temporary binder desirably volatilizes and exits the reactor.
The temporary bindery where used, desirably provides the aggregate with sufficient strength and physical integrity to withstand mechanical forces present during processing. In other words, the binder minimizes, if not eliminates, a tendency of an aggregate body to break apart when subjected to mechanical forces such as those generated when bodies rub against each other or reactor surfaces during processing.
An aggregate that breaks apart easily is particularly undesirable when using a fluidized bed reactor or a transport reactor. Breaking apart or "dusting" may lead to separation of the carbon from the metal oxide. This, in turn, impedes the reaction and may lead to an incomplete reaction product that contains an undesirable fraction of unreacted metal oxide. Dusting also typically leads to a reduced yield as very fine residue from broken aggregates is lost overhead in an apparatus such as a fluidized bed reactor. In addition, for systems with larger aggregates (e.g. fixed beds, moving fixed beds and rotary reactors) dusting impedes inert or reactant gas flow through a mass or bed of aggregate bodies. Gas flow may also be restricted to channels that form in the bed thereby leading to nonuniform flow of gas through the bed. Furthermore, dusting can dramatically reduce radiative heat transfer in favor of increasing conductive heat transfer within the bed. The latter is much slower than the former and typically leads to a reactor throughput that is much lower than would be possible with only radiative heat transfer.